Floating construction, a platform construction, a method for placing a floating platform construction at sea, and a method for removing a platform construction at sea

ABSTRACT

A floating construction, includes two elongated rigid steel columns, which are rigidly connected to each other by a rigid steel cross connection, the length of the columns being at least 50 meters, and a clear passage with a width of at least 16 meters being bounded between the columns, while near one end each of the columns includes a buoyancy-providing chamber and near the opposite end each of the columns includes a ballast, and between those ends a fixing point for a position-determining connection, such as an anchor line, is present on each of the columns. The invention further relates to a platform construction includes such a floating construction, and also to a method for placing a floating platform construction at sea and a method for removing a platform construction at sea.

The present invention relates to a floating construction and a superstructure comprising such a floating construction, such as a platform construction for extracting natural resources.

There is an ever-increasing demand for natural resources, such as oil and gas, where the production is carried out in deep water with floating platforms. These places are subject to strong winds and high waves, which can result in unacceptable rocking, pitching and rolling of the platform construction. In addition, there are difficulties with the installation of the platform of an offshore platform construction. Floating in (placing the platform on the floating construction by means of ballasts) is not possible at sea when the height of the waves is one meter and higher as a result of the great forces caused by the relative movements between the platform and the floating construction. The platform constructions from the prior art are usually based on the so-called spar design, in the case of which a single large central floating column is connected to the center of a superstructure. There are designs in which this column also serves as an oil storage tank. A disadvantage of these spar designs is that the connection of platform construction and floating construction is based on supporting the floating construction with a vessel on either side. Another disadvantage is that the spar designs are often difficult to transport. Owing to its round shape, a spar is also adversely affected by movements caused by eddies around the body of the dike.

An object of the present invention is to solve the abovementioned problems. This is made possible by a floating construction comprising two elongated rigid steel columns, which are rigidly connected to each other by means of a rigid steel cross connection, the length of said columns being at least 50 meters, and a clear passage with a width of at least 16 meters being bounded between the columns, while near one end each of said columns comprises a buoyancy-providing chamber and near the opposite end each of said columns comprises a ballast, and between those ends a fixing point for a position-determining connection such as an anchor line, such as a fastening for screws in order to keep the platform dynamically in position, is present.

Such a floating construction makes it possible to sail a vessel or a pontoon between the two elongated rigid steel columns during the floating-in process.

The superstructure is placed on this vessel. Said superstructure is lifted off the vessel by the rising floating construction, after which the vessel can sail away without superstructure. Furthermore, the floating construction according to the invention makes it possible to transport the floating construction, preferably as a whole or in parts and preferably horizontal, to the final offshore location and to set up and connect the construction or parts of the construction there on site, or vice versa.

It has been found that the floating construction according to the invention exhibits good movement behavior in high waves and strong winds. A platform can even be installed in waves 2 to 3 meters high by floating in between the elongated columns. As a result of this the risk of loss of time for the installation because of inability to work becomes minimal and acceptable for all possible offshore oil fields.

In an embodiment of the floating construction according to the invention, connecting means may be present for connecting said columns to each other by way of the cross connection and for disconnecting them from each other if necessary. The advantage of this is that the floating construction can be transported to the offshore location horizontally in separate parts. At this location the columns are moved into the vertical position (set up). After the setting up the elongated columns are connected to each other by means of the rigid steel cross connection and by way of the connecting means, so that a stable and rigid construction is obtained.

Said columns are preferably provided on the top side with fixing means for rigid fixing to a superstructure, so that after the floating in a superstructure can be fixed rigidly to the floating construction.

In a preferred embodiment the columns comprise a liquid storage tank. Ballast tanks may also be provided in the columns. Instead of ballast tanks, a lattice construction can also be provided.

In another preferred embodiment the fixing points are provided in such a way that the line of action of the cables or chains or screws provided there extends through the overall center of gravity. The cross connections are preferably provided in such a way that they can absorb the forces of cables or chains.

The invention further relates to a platform construction comprising a floating construction according to the invention, and also a superstructure that is rigidly connected to each of the columns.

The platform construction is preferably provided with means for extracting petroleum. To this end, said platform is advantageously provided with a provision line, drill line, production line and/or export line.

Floating constructions and platform constructions are known, for example from the international patent application WO 02/35014, in which four buoy elements bear a platform and the buoy elements are connected to each other by way of a horizontal anchor line. U.S. Pat. No. 6,206,614 also describes a platform construction with a floating construction comprising four columns that are situated close to each other and are connected to each other by horizontal and vertical plates respectively.

The invention also relates to a method for placing a floating platform construction at sea, providing a floating construction comprising two elongated steel columns which are situated at a distance from each other and are placed in a substantially vertical position at the location where they are to be used, placing a superstructure horizontally on a vessel, sailing vessel in between said columns, raising said columns relative to the sea surface level for making contact with the superstructure, rigidly fixing said superstructure to said columns, and sailing away said vessel. This method provides a simple way of placing a floating platform construction, with the advantage that it is not necessary to sail round the construction, but that only one vessel provided with the superstructure is sailed in between said columns. The floating construction can be taken in as a whole, or in parts in the case of large constructions.

The provision of the floating construction preferably comprises horizontal transportation of columns, which may be placed horizontally behind a vessel, to the location for use. The provision of the floating construction preferably further comprises rigid connection of said columns, such as by way of a rigid cross connection, at the location for use. If the platform is supplied in parts to the offshore location, it is preferably provided with hinges and guide pins in order to facilitate the offshore assembly.

In a preferred embodiment of the method according to the invention the raising of the columns comprises feeding gas under pressure into a chamber of the columns during the expulsion of water from the chamber. The advantage here is that the means for raising the columns are integrated in the columns and no external means are necessary for this, and the connection can be made very quickly.

The invention further relates to the possibility for extension of the platform construction in the horizontal and in the vertical direction, for example, by connecting several constructions together.

The invention furthermore also relates to a method for removing a platform construction at sea, comprising two vertical columns, moving said columns toward each other from the outer periphery of the bearing construction of said platform construction, making a rigid cross connection between said columns which connects said columns rigidly to each other, and raising said columns on contact being made by said floating construction with said platform construction. This method provides a simple method for taking over a superstructure from a bearing construction connected to it by the floating construction making contact, so that the superstructure and the bearing construction can be removed easily.

The invention will be explained in greater detail below on the basis of the description of a number of preferred embodiments, which are shown in the appended figures, in which:

FIG. 1 shows diagrammatically and in perspective a platform construction with two elongated columns, which are preferably mobilized in one unit;

FIG. 2 shows diagrammatically and in perspective a platform construction with four elongated columns, which are preferably mobilized in one unit;

FIG. 3 shows a platform construction which is supplied in parts, with hinges for positioning the connections, in the case of which the connections can be made above water;

FIG. 4 shows diagrammatically a side view of a platform construction in which it is indicated which part may be replaced by a lattice construction;

FIG. 5 shows diagrammatically the aids for positioning the construction parts of the floating construction;

FIG. 6 shows guide pins for connecting the cross connection;

FIG. 7 a shows a hinge as an underwater connection or as a means for positioning;

FIG. 7 b shows the hinge of FIG. 7 a with partially ballasted tanks;

FIG. 7 c shows the hinge in the transport position and in the connected state;

FIG. 8 shows diagrammatically and in perspective view a superstructure being floated in between the columns by ship;

FIG. 9 shows diagrammatically and in perspective view a floating superstructure being floated in between the columns;

FIG. 10 shows how the platform construction can be extended in the horizontal direction;

FIG. 11 shows how the platform construction can be extended in the vertical direction;

FIG. 12 illustrates the situation after assembly;

FIGS. 13 a to 13 c show diagrammatically the three steps for raising the columns relative to the sea surface level for making contact with a superstructure; and

FIG. 14 shows diagrammatically the layout of a floating construction for removing a platform construction;

FIG. 1 shows a platform construction 5 comprising a floating construction 1, which is constructed from two elongated rigid steel columns 3, which are rigidly connected to each other by rigid steel cross connections 4. The columns 3 are each rigidly connected to a superstructure 2. The length of the columns is at least 50 meters, but in the case of heavier platforms lengths of over 200 meters can also be found. The clear passage between the columns has a width of at least 16 meters, in order to permit a vessel to sail in. Depending on the size and weight of a platform, this clear passage can also be wider than 90 meters. The superstructure 2 is preferably an oil extraction platform or gas extraction platform. In the embodiment of FIG. 1 two rigid cross connections 4 are provided, but according to the invention it is also possible to limit this figure to one, or to provide more cross connections. Near the end that is closest to the surface of the sea each column is provided with a buoyancy-providing chamber, and near the opposite end each column is provided with a ballast (not shown). The superstructure 2 in general has dimensions of at least 15×15 meters and can vary in weight between 800 tonnes and 120 000 tonnes. The columns project with a so-called freeboard above the sea surface. The floating construction 1 is preferably brought in as a whole unit.

FIG. 2 shows another embodiment of a platform construction, in which a floating construction 6 comprises four elongated rigid steel columns 7, which are connected to each other by rigid steel cross connections 8. FIG. 2 also shows one of the lines from and to the platform connecting with a provision on the seabed 90. The connecting provision 10 lies below the platform if they are being installed from the platform itself. This is indicated by dotted lines in FIG. 2. The platform construction is connected to the seabed by means of anchor lines 12, and the anchor line can be fixed by means of a gravity anchor, a suction anchor or a pile foundation (not shown). The anchor lines 12 are fixed to the floating construction 6 at fixing points 11, which are provided near the cross connections. Means for the extraction of gas and oil 13 are provided on the platform.

FIG. 3 shows a solution for large platform constructions in the case of which the floating construction is divided at positions 63 and 64, with two hinges being provided at positions 63. Positions 63 and 64 can be brought above water in order to make the rigid connection.

FIG. 4 shows diagrammatically a side view of the floating construction, in which, apart from the known parts from FIGS. 1 and 2, the locations are indicated respectively of a chamber 15 filled with air for providing buoyancy, a chamber 16 filled with water as a temporary ballast and a chamber 17 filled with a permanent or changeable ballast such as iron ore. The ballast tanks 16 can also be replaced by a lattice construction.

FIG. 5 serves to illustrate the positioning of the individual construction parts 18. For the setting up, in this case a forerunner 22 is connected from a top cross connection to a pulley 23 or a fixed point on the bottom cross connection. The forerunners provided beforehand are connected above water. The two construction parts 18, 21 can then be connected by hauling in the forerunners by means of a winch 24, which is situated on, for example, a support vessel.

FIG. 6 shows a preferred embodiment of the connecting means for connecting the columns to each other by way of the cross connection. For this purpose a guide pin 25 is provided, which guide pin has a loose fit with an interacting guide cone 26 situated opposite it. A spacing ring 27 is further provided around the guide pin 25, which spacing ring has a narrow tolerance with the guide cone 26 and is aligned by means of epoxy or an equivalent material. The guide pin 25 is connected to the cross connections 4, 8. The guide cone 26 and spacing ring 27 have a close fit and are fixed to the cross connections. Various connecting techniques can be used when the parts are in position.

FIGS. 7 a and 7 b show how the platform parts can be connected by means of a hinge 79 and how the parts can be brought together by ballasting of tanks 80.

FIG. 7 c shows in detail an embodiment of this connection in which the hinge 79 and shearing connection 78 transfer the load.

FIGS. 8 and 9 show respectively the floating in of a platform 2 by means of a vessel 30 (FIG. 8) or the floating in of a floating deck 31 (FIG. 9). When the floating construction has been set up and the elongated columns 7 have been connected, the superstructure 2 or the floating deck can be floated between the columns, after which the columns are raised relative to the sea surface level by removing the ballasting. The columns rise to make contact with the superstructure 2 or the floating deck 31, followed by the rigid connection to the columns 7. In the case of the platform the vessel 30 can then sail away again from the platform construction formed. Elastic buffer blocks 111 and stoppers 112 can be provided between ship and floating construction or floating superstructure and floating construction, in order to assist with positioning.

FIG. 10 shows how the floating construction can be extended in the horizontal plane by connecting parts 82. The mooring system can be designed in such a way that the construction can turn, so that the mooring forces are reduced. The connected situation is shown.

FIG. 11 shows how the platform can be extended in the vertical direction by connecting sections 83.

FIG. 12 shows the connected situation.

FIGS. 13 a to 13 c show the three steps for the transfer of a platform to a floating construction 1, 6. In a first step, referred to as the first contact phase, water is driven out of a chamber by using gas under pressure, such as compressed air. In a second step, referred to as the load transfer phase, the weight of the platform is transferred to the columns, in general a gap 33 at first still being present between a column and the platform. Positioning pins 34 are preferably used for good positioning between the platform 2 and the columns. When the columns rise yet further, in a third step, referred to as the release phase, a gap 35 is created between a vessel or a pontoon 36 and the platform 2. Finally, the deck is lifted fully from the pontoon or the ship, which can then sail away. In a final step (not shown) the platform 2 is rigidly connected to the columns of the floating construction 1, 6.

FIG. 14 is intended to illustrate a method for removing a platform construction at sea, in the case of which columns 7 of a floating construction 6 are moved towards each other from the outer periphery of the bearing construction 46 of the platform construction in the direction of the arrows A. A rigid cross connection, which connects the columns rigidly to each other, is subsequently provided between the columns 7. Raising the columns causes supporting beams 40 of the floating construction 6 to make contact with the superstructure 2 of the platform construction. After the connections with the bearing construction 46 have been broken, the platform 2 can be lifted. The platform 2 is placed on a pontoon by ballasting the latter in the downward direction. By means of lifting bars 39, which are operated by means of hoisting wires 38, the bearing construction 46 can be dismantled into loose parts and the parts cut off in each case can be transferred to a pontoon. The cross connections 8 of the floating construction 6 can be adapted in length by installing or removing intermediate sections 37. In this way platforms of various sizes can be removed.

It is also possible by means of the invention to carry out the operations described above in rough seas.

It must be understood that the term superstructure can mean any desired structure on the floating construction, such as—but not restricted to—jetty, landing stage and an offshore base for oceanographic research.

Of course, the details of the constructions and embodiments shown can vary widely without departing from the principle of the invention, as illustrated in the appended claims. 

1-13. (canceled)
 14. A floating construction comprising two elongated rigid steel columns, which are rigidly connected to each other by a rigid steel cross connection, the length of said columns being at least 50 meters and a clear passage with a width of at least 16 meters being bounded between the columns, while near one end each of said columns comprises a buoyancy-providing chamber, and near the opposite end each of said columns comprises a ballast, and between those ends a fixing point for a position-determining connection, such as an anchor line, is present on each of said columns.
 15. The floating construction as claimed in claim 14, in which connecting means are present for connecting said columns to each other by way of said cross connection and for disconnecting them from each other.
 16. The floating construction as claimed in claim 14, in which said columns are provided at one end with fixing means for rigid fixing to a superstructure.
 17. The floating construction as claimed in claim 14, in which said columns comprise a liquid storage tank.
 18. A platform construction comprising a floating construction as claimed in claim 14, and also a superstructure, which is rigidly connected to each of said columns.
 19. The platform construction as claimed in claim 18, in which said superstructure is provided with means for extracting petroleum.
 20. The platform construction as claimed in claim 19, comprising a provision line, drill line, production line, and/or export line extending from said superstructure.
 21. The platform construction as claimed in claim 18, in which said fixing points are provided in such a way that the line of action of the cables or chains or screws provided there extends through the overall center of gravity.
 22. A method for placing a floating platform construction at sea, comprising the provision of a floating construction comprising two elongated steel columns which are situated at a distance from each other and are placed in a substantially vertical position at the location where they are to be used, placing a superstructure horizontally on a vessel, sailing the vessel in between said columns, raising said columns relative to the sea surface level o making contact with said superstructure, rigidly fixing th superstructure to said columns, and sailing away said vessel.
 23. The method as claimed in claim 22, in which the provision of said floating construction, preferably comprises horizontal transportation of the columns to the location for use.
 24. The method as claimed in claim 22, in which the provision of the floating construction comprising rigid connection of said columns, such as by way of a rigid cross connection, at the location for use.
 25. The method as claimed in claim 22, in which the raising of said columns comprises feeding gas under pressure into a chamber of said columns during the expulsion of water from the chamber.
 26. A method for removing a platform construction at sea, comprising providing a floating construction comprising two vertical columns, moving said columns toward each other from the outer periphery of the bearing construction of said platform construction, making a rigid cross connection between said columns which connects said columns rigidly to each other, and raising said columns on contact being made by said floating construction with said platform construction. 